Many electrical devices, such as cell phones, personal digital assistants (PDA's), laptops, etc., use power to operate. Because power is generally delivered through a wall socket as high voltage alternating current (ac), a device, typically referred to as a power converter, can be used to transform the high voltage ac input to a well regulated direct current (dc) output through an energy transfer element. Switched mode power converters are commonly used due to their high efficiency, small size, and low weight to power many of today's electronics. In a typical operation, a switched mode power converter uses a switch to provide the desired output quantity by either varying the duty ratio (typically the ratio of the on-time of the switch to the total switching period) or by varying the switching frequency of the switch. Among the various switched mode power converter topologies, a flyback converter is a commonly used topology for low-cost power converters. In a typical application, the ac-dc power converter receives an input from an ordinary ac electrical outlet. The output of the power converter is typically a dc voltage, but may be a regulated dc current for applications such as charging batteries.
Safety agencies generally require the power converter to provide galvanic isolation between input and output. Galvanic isolation prevents dc current between input and output of the power converter. In other words, a dc voltage applied between an input terminal and an output terminal of the power converter will produce no substantial dc current between the input terminal and the output terminal of the power converter. The requirement for galvanic isolation may be a complication that contributes to the cost of the power converter.
A power converter with galvanic isolation maintains an isolation barrier that electrically separates the input from the output. Energy is transferred across the isolation barrier to provide power to the output, and information in the form of signals is transferred across the isolation barrier to regulate the output. Galvanic isolation is typically achieved with electromagnetic and electro-optical devices. Electromagnetic devices such as transformers and coupled inductors are generally used to transfer energy from input to output to provide output power, whereas electro-optical devices are generally used to transfer signals from output to input to control the transfer of energy from input to output.
Efforts to reduce the cost of the power converter have focused on the elimination of electro-optical devices and their associated circuits. Alternative solutions generally use an energy transfer element such as a transformer or a coupled inductor to provide energy to the output and also to obtain the information necessary to control the output. One low cost configuration places the control circuit and a high voltage switch on the input side of the isolation barrier. The controller obtains information about the output indirectly from observation of a voltage at a winding of the energy transfer element. The winding that provides the information is also on the input side of the isolation barrier.
The information about the output received in an indirect manner as described above is based substantially on the magnetic coupling between the windings placed on the input and the output sides of the energy transfer element. The magnetic coupling between the windings may not be perfect due to physical and mechanical limitations associated with the placement of the windings of the energy transfer element. This may lead to inaccurate information about the output which may further lead to poor regulation of the output. Therefore, generating more reliable information about the output would improve regulation of power converter outputs.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.